Electrical readout integrator having porous insulators and spaced electrodes in an electrolyte



5, 1966 c. w. HEWLETT, JR 3,289,677

ELECTRICAL READOUT INTEGRATOR HAVING POROUS INSULATORS AND SPACEDELECTRODES IN AN ELECTROLYTE Filed March 29. 1963 TNVFNTOR.

CLARENCE W. HEWLETT JR H IS ATTORNEY United States Patent York FiledMar. 29, 1963, Ser. No. 269,037 14 Claims. (Cl. 317-231) This inventionrelates to electrochemical devices, and more particularly, to animproved solion.

A solion is a precision electrochemical cell utilizing an electrolyticsolution which enters into a reversible electrochemical reaction at itselectrodes under preselected operating conditions. One of thecharacteristics of such devices which makes them extremely useful istheir ability to reliably integrate applied electrical signals.

Such devices are characterized by the separation of the container by itselectrodes into two compartments separated by a barrier space. The firstcompartment is termed the reservoir and is defined by an input electrodeand a shield electrode, while the second, or integral compartment, isdefined by the readout electrode and a common electrode. The space thusformed between the readout and shield electrode is referred to as thebarrier space.

As is well known, the chemical reaction which takes place in one form ofsolion device may be the result of the reduction of one species of theions in solution at one of the electrodes and the correspondingoxidation of the other species of the ion at another electrode of thesystem. This redox system must be completely reversible in a solion inorder for it to accurately perform its integrating function. This willobtain as long as the applied voltages stay below the potential at whichthe ions of the solution react with the electrodes to generate a gaseousby-product that will escape from the solution. Thus, the electrodes andthe electrolyte of a solion are so chosen that at their operatingpotentials they do not enter into an electrochemical reaction which isirreversible. Thus, the electrochemical reaction results in theinterchange of electrons between the ions in solution and the electrodesto thus change the signal from the form in which it is represented byelectron conduction in a metallic conductor to the form in 'which it isrepresented by ionic conduction in the electrolytic solution.

A common electrolyte for the solion redox system is an aqueous solutioncontaining a small amount of iodine and a comparatively large amount ofpotassium iodide. This results in a solution containing iodide ions andtriiodide ions, the latter of which will hereinafter be referred to :asiodine. In such a redox system, iodine becomes What has been termed themeasured species, while the iodide ion is the unmeasured species of theion of the solution. If the signal to be integrated is applied betweenthe common and input electrodes, while the common electrode is positivewith respect to the input electrode, the concentration of iodine willincrease at the common electrode. If a potential is now placed acrossthe integral compartment by making the common electrode positive withrespect to the readout electrode, a current will flow Which isproportional to the amount of iodine within the integral compartment.Thus, the read-out current will be proportional to the chargetransferred between the input and common electrodes thus giving thedesired integration function.

A solion is a precision electrochemical device which necessitatesextreme care in its manufacture in order to obtain a reliable devicewith desirable operational characteristics. Operational reliability is asignificant factor to be considered in the construction and design of asolion due to the precise spacing necessary between various elements ofthe device which renders it susceptible to both open circuits andshorts. Such operational problems have necessitated the exercise ofgreat care in the design and assembly of the solion devices thusrendering them expensive and difiicult to produce on a production linebasis without encountering a high rate of rejects. The production ofprior art solions has thus been, of necessity, essentially a handassembly operation for a highly skilled operator.

It is therefore :an object of the invention to simplify the constructionof solions by minimizing the number of necessary components, as well asmi-nimizingthe number and criticality of the steps necessary in itsassembly.

A great deal of effort has also been expended in designing andconstructing solions so as to improve their operational characteristics.Two of the more important areas of eifort relate to the improvement ofresponse time or frequency response, as well as the minimization ofdrift rate. The response time has to do with the length of time after acharge has been introduced before the readout current will respond tothis charge. This time should,

. of course, be minimized in order to increase the frequency response ofthe unit.

The drift rate, on the other hand, effectively controls the length oftime that an integral can be stored in the compartment with any givendegree of accuracy. The drift of a unit is evidenced by a long termchange-of the readout current when a given integral is being stored inthe device. This phenomenon is due to the diffusion of the iodine ormeasured species across the barrier space.

It is therefore another object of the invention to provide a simplifiedsolion which is easier to construct, while at the same time displayingimproved drift characteristics.

It is a further object of this invention to provide a .solion whereinthe accurate control of critical spacing of elements is readilyfacilitated and dimensional stability is thereafter maintained due toits manner of construction.

It is yet another object of this invention to provide a solion in whichthe electrical connections to the electrodes are easily obtained duringthe assembly of the device without requiring time consuming and criticaloperations.

It is still another object of this invention to provide a solion inwhich the integral compartment is defined by electrodes which havesubstantially planar surfaces which facilitate the extremely closeparallel alignment of the electrodes to provide a fast response time andcorrespondingly high frequency response.

Briefly described, this invention contemplates stacking two glass fritdisks, each of which bear platinum electrodes; two annular conductiverings; and a third platinum electrode within a glass cup-shapedcontainer, the bottom of which contains a fourth electrode molded in itsbottom. Upon stacking the previously-noted elements in their properorder within the cup-shaped container, a glass top is inserted withinthe open top of the cup-shaped container and brought into position incontact with the top of the stack. The top is then sealed in thisposition and the open tube, integral with the top which provides accessto the interior of the container, may then be utilized to evacuate thecontainer when it is under heat so that the softened container isbrought into intimate contact with the stacked elements. Thereafter, thetube may be used for filling the container with the solution after whichit is sealed off to complete the unit and also provide an expansionchamber in the end of the tube. V

The subject matter of the invention is particularly pointed out anddistinctly claimed in the'concluding portion of this specification. Theinvention, however, both as to organization and method of operation,together with further objects and advantages thereof, may best beunderstood by reference to the following description taken in connectionwith the accompanying drawing which is a cross-sectional and elevationalview of the preferred embodiment of this invention.

Referring now to the drawing, there is shown a solion device which maybe employed as an integrator. This solion integrator 10 includes acontainer consisting of glass cup-shaped portion 11a and top portion 11bwhich, when sealed together along surface 12, provides a sealedcontainer for the electrolytic solution 13.

One preferred glass suitable for the solion container 11 and the glassfrits is a chemically inert, heat resistant, low expansion glass denotedas Pyrex glass. One or the preferred types of Pyrex is commerciallyavailable as 7740 Pyrex glass. The use of Pyrex glass, plus theselection of platinum or its noble metal alloys for the electrodes,provides a device the elements of which are chemically inert to theelectrolyte and its solvent. One of the common redox systems now in usecontains iodine (I and potassium iodide (KI) in an aqueous solution withthe concentration of K1 usually about 100 times the concentration of IThis discussion will proceed with reference to the operation of thedevice of the invention with reference to a potassium iodide-iodineredox system, even though it should be recognized that the invention isequally applicable to other redox systems.

Lead 15, which is welded to portion 17 of common electrode 14, and readout electrode lead 16 are imbedded in cup-shaped portion 11a during itsmolding to provide means for establishing an externally availableconnection to the corresponding electrode. Common electrode 14, which isformed of platinum gauze of about 80 mesh, is also molded in cup-shapedportion 11a during the molding operation which can be carried out by aplunger and die assembly which simultaneously molds portion 11a whilepressing the electrode and leads in place.

In the preferred embodiment, for simplicity and accuracy, the die isformed so that the top surface of common electrode 14 is flush with theannular surrounding area with which spacer 20 is later brought incontact. Thus, the spacing between the electrodes of the integralcompartment is governed by the thickness of spacer 20. In addition,spacer 20 also provides a convenient manner for automaticallyestablishing a connection between lead 16 and readout electrode 21during the assembly operation. This type of construction readily lendsitself to changing the size of the integral compartment so as to obtaindesired operational characteristics merely by changing the thickness ofthe spacer used during the assembly operation.

Portion 17 is provided to stiffen common electrode 14 in cooperationwith L-shaped stiffening element 18 during the molding operation. Inaddition, portion 17 and element 18 facilitate the anchoring of controlelectrode 14 to container portion 11a upon completion of the moldingoperation.

It is an important feature of the invention that the gauze from whichcommon electrode 14 is formed has been flattened prior to the moldingoperation so that it presents a substantially planar surface with smallapertures therein for reasons which will hereinafter be more completelydiscussed.

Readout electrode 21 is, in turn, in the form of a sputtered layer ofplatinum placed upon the lower surface of disk-like glass frit insulator22. Shield electrode 24 is in like manner sputtered on the lower surfaceof disk-like glass frit 23. Platinum spacer 25, which is interposedbetween shield electrode 24 and glass frit 22 is 5 mils thick in thepreferred embodiment and provides means for completing a connectionbetween shield electrode '24 and shield lead 20.

Input electrode 26, which may also be composed of platinum gauze ofabout mesh, is in turn held in contact with the upper surface of disk 23due to the pressure exerted by collar 27 when top 11b is brought intoposition in contact with the stacked array of elements.

Notches are provided at two points around the periphery of collar 27 inorder to accommodate leads 19 and 20 which are welded to input electrode26 and shield-contacting annular ring 25, respectively. These notchesare filled during the flame sealing operation to secure a vacuum-tightjoint along this line.

Provisions have been made to prevent the leakage of solution aroundleads 15, 16, 19 and 20 which are connected to their correspondingelectrodes by passing through the wall of container 11. An effectiveseal between these leads and the glass container is provided by theplatinumtungsten-platinum butt-welded jointssince tungsten will bond tothe Pyrex glass, while platinum will not, thereby affording thenecessary seal. However, it should be recognized that externallyavailable tungsten leads may be utilized thus necessitating onlyplatinum-tungsten buttwelded joints within the walls on container 11b.

Referring now to the integral compartment formed between readoutelectrode 21 and common electrode 14, the response time of the solion,as well as its sensitivity, will be primarily determined by the distancebetween these two electrodes. The smaller the interelectrode spacingthat may be accurately maintained without shorting, the faster theresponse time and the higher will be the sensitivity attainable.

Heretofore, it has been the practice to use platinum gauze in itsconventional form for the common electrode since the apertures providedtherein aid in quickly achieving equilibrium of the measured species inthe inte gral compartment by providing fast access by the solutionbetween the common electrode and the supporting glass to the integratorcompartment. It has been found that further improvement in response timeand sensitivity of the prior art solions has been limited by theinability to place the electrodes any closer together without increasingthe incidence of shorts between the electrodes.

In accordance with the invention, significant improvement in responsetime and sensitivity is readily obtained without increasing theincidence of electrode shorting by applying large forces to flatten thegauze utilized so that it will provide a substantially planar electrodesurface. This accomplishes the desired results since the average of allthe interelectrode path lengths is decreased until it is of the sameorder of magnitude as the shortest path length. This may be seen whenyou consider that the shortest electrode path length in the averagesolion is of the order of a few mils; e.g., in the preferred embodiment,platinum spacer 20 is 2 mils thick, while the diameter of the wireforming the gauze is of the same order of magnitude when 80 mesh gauzeis utilized. Further-- more, since the gauze is normally formed byweaving the wire, the difference between the shortest path length andthe longest is correspondingly increased. These factors are importantsince the sensitivity and response time attainable with such devices is,in the final analysis, a function of the longest path length between theintegral compartment electrodes.

The interrelationship between interelectrode path length, response andsensitivity may be explained by the fact that when an input signal isapplied across the input and common leads iodine is deposited along thesurface of the common electrode when the common electrode is positivewith respect to the input electrode. This newly deposited iodine mustthen diffuse across the integral compartment and appear at the readoutelectrode before its presence can be evidenced by current flowing in thereadout circuit. Since the time constant of diffusion of the iodinebetween the common and readout electrodes varies as the square of thedistance between the electrodes, it can be seen that the presence ofsignificantly longer path lengths between certain portions of the commonelectrode and the readout electrode will greatly increase the responsetime of the unit.

To summarize, a precision electrochemical solion device is provided bymolding the common electrode of flattened gauze mesh into apredetermined position wherein it will cooperate with, and be properlyspaced from, a sputtered platinum readout electrode by stacking theglass frit disk bearing the readout electrode upon a platinum spacer.Thus, the extremely critical integral compartment is automaticallyformed without requiring any highly critical or time consuming steps oroperations. The solion device is thereafter completed by stacking theadditional elements, as well as the top portion of the container, withinthe open end of the cup. The assembly operation is then completed byapplying pressure to the stacked assembly and fiame sealing the top tothe body of the device.

Glass frits having pores in the range of .8 to 1.4 microns have beenfound to be successful in minimizing stirring when the unit is moved orvibrated. Platinum is sputtered on the glass frits to a thickness thatwill provide about a ohm resistance across the diameters of the shieldelectrode and the readout electrode. It is obvious that the sputteringprovides an electrode which is securely attached to the glass frit baseand has small pores therein to further prevent the leakage of themeasured species between the two compartments through the barrier space.

The following dimensions have been found to provide a suitable soliondevice with the previously noted ad- I vantages in construction andimproved operational characteristics. The outer diameter of cup-shapedportion 11a is of the order of 13 millimeters, while its inner diameteris about 10 millimeters. The outer diameter of the tube 11b isapproximately 6.6 millimeters, while its inner diameter is 3.3millimeters. The glass frits are of the thickness of 1.7 millimeters andhaving an outer diameter which matches the inner diameter of cupshapedportion 11a.

While there has been described what is at present considered to be apreferred embodiment of the invention, it will be apparent to thoseskilled in the art that various changes and modifications may be madetherein without departing from the spirit or scope of the invention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. An electrical readout integrator comprising a redox electrolyticsolution, a container for said electrolytic solution; a plurality ofspaced electrodes arranged within said container, said pluralitycomprising an input electrode, a shield electrode, a readout electrodeand a common electrode; first porous insulating means interposed betweenand in contact with said input and said shield electrodes, said inputand shield electrodes defining a reservoir compartment therebetween,said shield electrode being deposited on one planar surface of saidfirst porous insulating mean; second porous insulating means interposedbetween said shield and said readout electrodes,

'said readout electrode being deposited on one planar surface of saidsecond porous insulating means; first electrically conductive meansinterposed between and in contact with said shield electrode and saidsecond in: sulating means for establishing a connection to said shieldelectrode; second electrically conductive means for spacing apart saidreadout and common electrodes to define an integral compartmenttherebetween; and a plurality of conductor means connected to said inputelectrode, said first electrically conductive means, said secondelectrically conductive means and said common electrode for establishingexternally available connections to said electrodes, said electrodesbeing taken from the class consisting of platinum and its noble metalalloys.

2. An electrical readout integrator comprising a redox electrolyticsolution, a container for said electrolytic solution; a plurality ofspaced electrodes coaxially arranged within said container, saidplurality comprising an input electrode, a shield electrode, a readoutelectrode and a common electrode; first porous insulating meansinterposed between and in contact with said input and shield electrodesand in contact with said container, said input and shield electrodesdefining a reservoir compartment therebetween, said shield electrodebeing deposited on one planar surface of said first porous insulatingmeans; second porous insulating means interposed between said shield andreadout electrodes and in contact with said container, said readoutelectrode being deposited on one planar surface of said second porousinsulating means; said first and second porous insulating means beingdisk-shaped and arranged within said container coaxially with respect tothe axis of said electrodes; first electrically conductive meansinterposed between and in contact with said shield electrode and saidsecond insulating means and in contact with said container forestablishing a connection to said shield electrode; second electricallyconductive means for spacing apart said readout and common electrodes todefine an integral compartment therebetween; and a plurality ofconductor means connected to said input electrode, said firstelectrically conductive means, said second electrically conductive meansand said common electrode for establishing extern ally availableconnections to said electrodes, said electrodes being taken from theclass consisting of platinum and its noble metal alloys.

3. An electrical readout integrator comprising a redox electrolyticsolution, a container for said electrolytic solution; a plurality ofspaced electrodes coaxially arranged within said container, saidplurality comprising an input electrode, a shield electrode, a readoutelectrode and a common electrode; first and second spaced disklikeporous insulators arranged within said container coaxially with respectto the axis of said electrodes; said first porous insulator beinginterposed between and in contact with said input and shield electrodesand in contact with said container to define a reservoir compartmenttherebetween, said shield electrode being deposited on one planarsurface of said first insulator; said second porous insulator beinginterposed between said shield and said readout electrodes, in contactwith said container and said readout electrode being deposited on oneplanar surface of said second insulator; first electrically conductivemeans interposed between and in contact with said shield electrode andsaid second insulator for establishing a connection to said shieldelectrode; a conductor passing through the wall of said container andterminating at an inner surface of said container, second electricallyconductive means interposed between said read out electrode and saidcontainer for spacing said readout electrode from an end of saidcontainer, said common electrode being supported in a fixed positionwith respect to said end to define an integral compartment incooperation with said readout electrode, said second electricallyconductive means being in contact with said conductor for establishing aconnection between said conductor and said readout electrode; meansconnected to said first conductive means for establishing an externallyavailable connection to said shield electrode and first and secondconductor means connected to said input and common electrodes forestablishing externally available connections thereto, said electrodesbeing taken from the class consisting of platinum and its noble metalalloys.

4. An electrical readout integrator comprising a redox electrolyticsolution, a container for said electrolytic solution; a plurality ofspaced electrodes coaxially arranged within said container, saidplurality comprising an input electrode, a shield .electrode, a readoutelectrode and a common electrode; first and .second disklike porousinsulators arranged within said container coaxially with respect to theaxis of said electrodes; said first porous insulator being interposedbetween and in contact with said input and shield electrodes and incontact with said container to define a reservoir compartmenttherebetween; said second porous insulator being interposed between saidshield and said readout electrodes and in contact with said container;said readout electrode being in contact with said second insulator; saidelectrodes and said insulators being shaped to have a cross-sectionsubstantially identical to the cross-section of the interior of saidcontainer to prevent the passage of said solution between compartmentsby passing around said electrodes and said insulators; an annularconductive disk the periphery of which is substantially identical tosaid interior cross-section; said annular disk being interposed betweenand in contact with said shield electrode and said second insulator forestablishing a connection to said shield electrode; means for spacingapart said readout and common electrodes to define an integralcompartment therebetween; and a conductor passing through the wall ofsaid vessel and terminating on said annular conductive disk forestablishing a connection to said shield electrode and a plurality ofconductor means connected to said input, readout and common electrodesfor establishing externally available connections thereto, saidelectrodes being taken from the class consisting of platinum and itsnoble metal alloys.

5. An electrical readout integrator comprising a redox electrolyticsolution, a container for said electrolytic solution; a plurality ofspaced electrodes coaxially arranged within said container, saidplurality comprising an input electrode, a shield electrode, a readoutelectrode and a common electrode; first and second spaced disk-likeporous insulators arranged within said container coaxially with respectto the axis of said electrodes; said first porous insulator beinginterposed between and in contact with said input and shield electrodesand in contact with said container to define a reservoir compartmenttherebetween; said second porous insulator being interposed between saidshield and said readout electrodes and in contact with said container;said readout electrode being in contact with said second insulator; saidelectrodes and said insulators being shaped to have a cross-sectionsubstantially identical to the cross-section of the interior of saidcontainer to prevent the passage of said solution between compartmentsby passing around said electrodes and said insulators; an annularconductive disk the periphery of which is substantially identical tosaid interior cross-section; said annular disk being interposed betweenand in contact with said shield electrode and said second insulator forestablishing a connection to said shield electrodes; a first conductorpassing through the wall of said container and terminating at an innersurface of said container, 21 second annular conductive disk theperiphery of which is substantially identical to said interiorcross-section; said second annular disk being interposed between saidreadout electrode and said container for spacing apart said readoutelectrode and an end of said container, said common electrode beingsupported in a fixed position with respect to said end to define anintegral compartment in cooperation with said readout electrode; saidsecond annular disk being in contact with said conductor forestablishing a connection between said conductor and said readoutelectrode; a second conductor passing through said container and beingelectrically connected to said first annular disk for establishing aconnection to said shield electrode and third and fourth conductorspassing through said container and being electrically connected to saidinput and common electrodes for establishing externally availableconnections thereto, said electrodes being taken from the classconsisting of platinum and its noble metal alloys.

6. The combination of claim in which said common electlfidt i5 SLJI91 I9and :in intimate contact with said end of said container, said commonelectrode having minute apertures therein whereby all of the ions insolution in said integral compartment are free to engage in ionicconduction.

7. The combination of claim 6 in which said common electrode comprises acup-shaped element of gauze, the gauze having been flattened by theapplication of a large force so as to present a substantially planarsurface with small apertures therein, said cup-shaped element beingmolded in said end of said container so that only the bottom of saidcup-shaped element is exposed to operate as the active portion of saidcommon electrodes.

8. The combination of claim 7 in which the outer periphery of the activeportion of said common electrode terminates short of the line of contactof the inner periphery of said second annular disk with said end.

9. The combination of claim 8 in which said container is Pyrex glass andsaid porous insulators are glass frits.

10. The combination of claim 9 in which said shield and readoutelectrodes are respectively sputtered on a planar surface of said firstand second glass frit disks.

11. The combination of claim 10 in which said input electrode is gauze,said gauze being retained in contact.

with said first glass frit disk by being interposed between and incontact with said first glass frit disk and an inner surface of thecontainer opposed to said end, said opposed inner surface terminating inan expansion chamber which is in direct communication with a centralportion of said input electrode.

12. The combination of claim 11 in which said electrolyte comprises anaqueous solution of potassium iodide and iodine.

13. An electrical readout integrator comprising a container having aportion with a generally cylindrical interior containing an electrolyticsolution; first and fourth electrodes in physical contact with theopposed ends of said sylindrical portion, a first porous glass frit diskhaving a second electrode in contact with one of its surfaces, thesurface of said first disk which is opposed to said one surface being incontact with said first electrode, a second porous glass frit diskhaving a third electrode in contact with one of its surfaces, thesurface of said second disk which is opposed to said one surface beingin spaced parallel alignment with said second electrode, said spacebetween said second disk and said second electrode being solely filledwith said solution except for the peripheral region of said space; meansfor spacing said third electrode in parallel alignment with said fourthelectrode; and means for establishing externally available electricalconnections for each of said electrodes, said electrodes being takenfrom the class consisting of platinum and its noble metal alloys.

14. An electrical readout integrator comprising a redox electrolyticsolution, a container for said electrolytic solution, a plurality ofspaced electrodes arranged within said container, said pluralitycomprising an input electrode; a shield electrode; a readout electrodeand a common electrode; means for supporting said input and shieldelectrodes in generally parallel alignment within said container todefine a reservoir compartment, a disklike porous insulator supportedwithin said container interposed between said input and shieldelectrodes to limit diffusion from said reservoir compartment; means forsupporting said readout electrode in generally parallel alignment withsaid shield electrode to define a barrier compartment; and means forsupporting said common electrode in spaced parallel alignment with saidreadout electrode to define an integral compartment, said commonelectrode supporting means comprising means for establishing anelectrical connection to said readout electrode and means forestablishing externally available connections for each of saidelectrodes, said electrodes being taken from the class consisting ofplatinum and its noble metal alloys, said common electrode being formedof a metallic gauze which has been flattened to provide a 2,661,430substantially planar surface while still retaining apertures 2,890,414therein. 3,021,482 References Cited by the Examiner 3,163,806

UNITED STATES PATENTS 5 3,211,967

2,178,969 11/1939 Ruben 3l7-230 2,644,902 7/1953 Hardway 317231 10Hardway 317-231 Snavely 3 17231 Estes 317-231 Estes et a1. 317231 Gramset a1. 317231 JOHN W. HUCKERT, Primary Examiner.

JAMES D. KALLAM, Examiner.

1. AN ELECTRICAL READOUT INTEGRATOR COMPRISING A REDOX ELECTROLYTICSOLUTION, A CONTAINER FOR SAID ELECTROLYTIC SOLUTION; A PLURALITY OFSPACED ELECTRODES ARRANGED WITHIN SAID CONTAINER, SAID PLURALITYCOMPRISING AN INPUT ELECTRODE, A SHIELD ELECTRODE, A READOUT ELECTRODEAND A COMMON ELECTRODE; FIRST POROUS INSULATING MEANS INTERPOSED BETWEENAND IN CONTACT WITH SAID INPUT AND SAID SHIELD ELECTRODES, SAID INPUTAND SHIELD ELECTRODES DEFINING A RESERVOIR COMPARTMENT THEREBETWEEN,SAID SHIELD ELECTRODE BEING DEPOSITED ON ONE PLANAR SURFACE OF SAIDFIRST POROUS INSULATING MEANS; SECOND POROUS INSULATING MEANS INTERPOSEDBETWEEN SAID SHIELD AND SAID READOUT ELECTRODES, SAID READOUT ELECTRODEBEING DEPOSITED ON ONE PLANAR SURFACE OF SAID SECOND POROUS INSULATINGMEANS; FIRST ELECTRICALLY CONDUCTIVE MEANS INTERPOSED BETWEEN AND INCONTACT WITH SAID SHIELD ELECTRODE AND SAID SECOND INSULATING MEANS FORESTABLISHING A CONNECTION TO SAID SHIELD ELECTRODE; SECOND ELECTRICALLYCONDUCTIVE MEANS FOR SPACING APART SAID READOUT AND COMMON ELECTRODES TODEFINE AN INTEGRAL COMPARTMENT THEREBETWEEN; AND A PLURALITY OFCONDUCTOR MEANS CONNECTED TO SAID INPUT ELECTRODE, SAID FIRSTELECTRICALLY CONDUCTIVE MEANS, SAID SECOND ELECTRICALLY CONDUCTIVE MEANSAND SAID COMMON ELECTRODE FOR ESTABLISHING EXTERNALLY AVAILABLECONNECTIONS TO SAID ELECTRODES, SAID ELECTRODES BEING TAKEN FROM THECLASS CONSISTING OF PLATINUM AND ITS NOBLE METAL ALLOYS.